Sound reproducer



Oct. 28, 1958 w. s. BACHMAN 2,858 370 SOUND REPRODUCER Filed Sept. 30, 1954 INVENTOR. WILLIAM S. BACHMAN BY PM W WA; 7 WM PTOAIIQL/ ATTORNEYS United States Patent SOUND REPRODUCER William S. Bachman, Southport, Conn., assignor to Columbia Broadcasting System, Inc., New York, N. Y., a corporation of New York Application September 30, 1954, Serial No. 459,335

6 Claims. (Cl. 179-1) This invention relates to amplifiers for audio frequencies and more particularly to a single stage vacuum tube audio amplifier adapted to drive a transformer-coupled loud-speaker from a phonograph record pickup.

There is a large demand for phonograph record players of low price, in which it is desirable forreasons of economy to use a single stage of amplification. In order however for such a device to drive a loud-speaker at a satisfactory level, it requires a substantial input signal, of the order of magnitude of several volts for example. Input signals of this magnitude have been hitherto provided by means of stiff crystal pickups which were provided with a heavy stylus pressure in order to generate substantial voltages. This solution is unsatisfactory for the playing of microgroove records which must be used with a force on the stylus of the order of or grams or less.

The present invention provides a single stage amplifier capable of driving a loud-speaker from a compliant crystal pickup generating voltages of the order of one volt. The amplifier of the invention employs regeneration between the secondary winding of the output transformer which feeds the speaker and the input circuit to the tube, combined with a loading of the transformer primary which precludes the development of oscillations and which at the same time furnishes adesirable negative feedback between the output and input circuits of the tube. The characteristics of the transformer loading which prevent the development of oscillations in the upper audio range where they would otherwise occur make the negative feedback effective in the same frequency range and thus provide the amplifier of the invention with a desirable tone control. According to another feature of the invention the elements which load the transformer primary are connected to the input circuit of the tube in series with a volume control whereby the tone control is made eifective primarily at low volume levels. Otherwise stated, the attenuation of high frequency components due to the operation of the negative feedback, which is primarily effective as to such components, operates only at low volume levels.' In this way there is obviated a disadvantage whereby at high input signal levels the amplifier when subject to the usual tone control overloads with the low frequency components, and gives very little sound in the middle and upper audio range to which the ear is most sensitive.

The invention will now be described in detail in connection with the accompanying drawing in which the single figure of drawings is a schematic diagram of an embodiment of the amplifier of the invention.

In the single figure of drawings a voltage generator 2, which may typically take the form of a crystal phonograph record pickup, is connected to drive a loud-speaker 4 through a single stage of vacuum tube amplification employing a tube 6. The signal developed by the generator 2 is applied across a volume control resistor 8 and a portion of a resistor 10, the latter of which serves both as an adjustable tone control and as part of a loaddrawings, the plate power supply takes the form of a i 2,858,370 Patented Oct. 28, 1958 ing circuit including in addition a condenser 12 effectively connected across the primary winding 14 of an output transformer generally indicated at 16. The control grid 18 of tube 6 is connected to an adjustable tap 9 on the volume control resistor 8, and the plate 20 is connected to one end of the transformer primary winding 14. The end of the resistor 8 remote from the pickup connects with a tap 11 on the resistor 10.

The tube is shown for illustrative purposes only as a pentode having a suppressor grid connected to the cathode and a screen grid connected to the source of positive. potential through a dropping resistor 22. Plate voltage for the tube is connected to the other terminal of the transformer primary and may be derived from any convenient source. In the exemplary circuit shown in the dry rectifier element 24 connected through a resistor 26 to one side of an A. C. power line 28. Filter condensers 30 are provided to smooth the output of rectifier 24.

Suitable bias for the tube is provided by a cathode biasing arrangement includinga cathode resistor 32 and a bypassing capacitor 34.

In accordance with the principles of the invention, the

output signal from the tube as developed across the primary of the transformer is regeneratively appliedto the input circuit in a suitably reduced proportion as it appears across the secondary winding 17. As indicated, the voltage across the transformer secondary winding is applied directly to the terminals of the voice coil 5 in speaker 4. One terminal of the transformer secondary is connected to chassis ground, and the other is connected to one terminal of the generator 2. The connections are selected with due regard to the polarity changes taking place in the transformer 16 so that when the plate 20 of the tube is on the negative half of a signal cycle, the terminal of the transformer secondary which is coupled back to the generator is positive with respect to the grounded terminal of the transformer secondary.

The voltage across the transformer secondary is there- 1 fore added to the signal voltage of the generator, as

applied to the series connection of resistors 8 and 10.

The turns ratio of the transformer 16 must be selected with two considerations in mind. On the one hand it must provide a proper impedance match between the voice coil of the speaker and the output circuit of the tube. On the other hand it must provide a voltage stepdown greater than the gain between the grid and plate of the tube. Otherwise the circuit will oscillate.

These requirements cannot be satisfied to the same extent at all frequencies by a simple selection of turns ratio. The nature of the speaker impedance varies with frequency; it typically shifts from capacitive to inductive as the frequency rises beyond some point in the middle audio range. This speaker impedance as reflected through the transformer into the output circuit of the tube alters the tube gain and gives rise to a possibility of oscillations even though the turns ratio is appropriately selected for lower frequencies. The development of oscillations is avoided according to the invention, with highly advantageous concomitant effects, by the provision of a capacitor with preferably a series loading resistor, the two elements being connected effectively across the transformer primary. Capacitor 12 is connected in series with resistor 10 between the plate 20 of the tube and a point 36 which is connected to chassis ground through a large capacitor 38. The capacitor 12 is selected of such size as to resonate with the loudspeaker inductance as reflected through the transformer at a frequency for which maximum gain of the entire circuit is desired, for example a frequency of the order of 3,000 cycles. The series resistor 10 reduces the sharpness of the response peak.

Connection, according to the invention, of the resistor or part thereof in the input circuit of pickup 2 and volume control resistor 8 introduces a negative feedback voltage in series with the pickup signal. Since the amount of feedback decreases with frequency below resonance, a bass compensation effect is obtained. Connection of the volume control resistor 8 to an adjustable tap 11 on the resistor 10 permits variation in the feedback voltage developed in resistor 10 actually employed in the input circuit, and hence provides an adjustable tone control, the effect of which, for any setting of tap 11, diminishes with increasing input setting at tap 9 on the volume control resistor where connection is made to the grid of the tube. This reduction in effectiveness of the tone control with increasing volume control setting is highly desirable in an amplifier of limited power which otherwise tends to overload with low frequencies without producing much apparent sound when the volume control is set for maximum input (tap 9 at the end of resistor 8 adjacent the pickup) with the tone control set for minimum high frequency components (tap 11 adjacent capacitor 36).

Iclaim:

1. A sound reproducer comprising a transducer, a vacuum tube including a cathode, control grid and. plate, a transformer having its primary winding connected in the plate-cathode circuit of said tube, and a loud-speaker having its voice coil connected in shunt with the secondary winding of said transformer, said secondary winding being connected for positive feedback of all audio signals in series with said transducer in the gridcathode circuit of said tube, said transformer having a primary to secondary winding turns ratio greater than the gain of said tube when operated into said voice coil through said transformer.

2. A sound reproducer comprising a transducer, a resistance connected in series with said transducer, a vacuum tube including a cathode, control grid and plate, a stepdown transformer having its primary winding connected in the plate-cathode circuit of said tube and its secondary winding connected in series with said transducer and resistance in the grid-cathode circuit of said tube, a loudspeaker having its voice coil connected in shunt with the secondary winding of the transformer, the connection of said secondary winding being such as to provide positive feedback of all audio signals in said grid-cathode circuit, and a capacitor coupled between said plate and said resistance, said capacitor resonating at a selected audio frequency with the loud-speaker voice coil inductance reflected through the transformer.

3. A sound reproducer: comprising a transducer, a vacuum tube including a cathode, control grid and plate,

a transformer having its primary Winding connected in the plate-cathode circuit of said tube and its secondary winding connected in series with said transducer, a loud-- speaker having its voice coil connectedv in shunt with said secondary Winding, a resistance connected with said transducer and secondary winding into a series circuit, one end of said resistance. being coupled tosaid cathode through a patch having low impedance to alternating voltages of audio frequency; means coupling a tap on said resistanceto the. grid ofsaid tube, and means coupling a capacitor between said plate and a point on said resistance between said tap and said one end of said resistance.

4. A sound reproducer comprising a transducer, two potentiometers, a vacuum tube including a plate, grid and cathode, a step-down transformer having a primary to secondary winding turns ratio greater than the gain of said tube, said transformer having its primary winding connected in the plate-cathode circuit of said tube and having its secondary winding connected for positive feedback with said transducer and potentiometers into a closed series circuit, one end of one of said potentiometers being connected to a tap on the other of said otentiometers, a loud-speaker having its voice coil connected in shunt with said secondary winding, means coupling said cathode to one end of the other of said potentiometers through a path of low impedance to alternating voltages of audio frequency, a capacitor coupled between the other end of said other potentiometer and said plate, and means to couple said grid to a tap on said one potentiometer;

5. A sound reproducer comprising a transducer, a vacuum tube including a cathode, control grid and plate, a transformer having its primary winding connected in the plate-cathode circuit of said tube and its secondary winding connected in series with said transducer for positive feedback, a loud-speaker having its voice coil connected in shunt with said secondary winding, a first variable resistance connected with said transducer and secondary winding into a series circuit, one end of said resistance being coupled to said cathode through a low impedance path for audio frequency voltages, means coupling a variable point on said resistance to the grid of said tube, and means including a second variable resistor varying in= versely in magnitude with the first variable resistor coupling a capacitor between said plate and a point on said first variable resistance between said variable point and said one end of said resistance.

6. A sound reproducer comprising a transducer, a vacuum tube including a cathode, control grid and plate, a transformer having its primary winding connected in the plate-cathode circuit of said tube and its secondary Winding connected for positive feedback in series with said transducer in the grid-cathode circuit of said tube, a resistance connected with said transducer and secondary winding into a series circuit, means coupling one end of said resistance with said cathode through a path of low impedance to. alternating voltages of audio frequency, a tap on said potentiometer coupled to said grid, means coupling a capacitor between said plate and a point on said resistance closer to said cathode than said tap, and a loud-speaker having its voice coil connected in shunt with said secondary winding, said capacitor being dimensioned to resonate with the inductance of said voice coil as reflected through said transformer at a frequency in the middle audio range.

References Cited in the file of this patent UNITED STATES PATENTS 

